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For most people, squeezing in enough time to exercise regularly is challenging, but 39-year-old Erin, who appeared on Tuesday’s episode of "The Doctors,” faces the opposite issue — she can’t stop working out.

In fact, as the feel-good effects of exercise have waned for the San Diego, California, woman, Erin has increased her workout to eight hours per day.

“I’ll cancel plans, I’ll cancel appointments. It’s been controlling my life,” she told Dr. Travis Stork, who hosts the show. “I just can’t stop. It’s not giving me the rush that I used to feel just doing three to four hours.”

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Although Erin is heading to rehab for exercise addiction and an eating disorder in April, her story highlights the scary and potentially life-threatening health effects of getting too much of a good thing, experts say.

“For most people, after you exercise, you feel better, you’re happier and you’re in a better mood, but for someone who’s exercise addicted, they’ll get depressed and they must get their exercise in,” Heather Hausenblas, a kinesiology professor at Jacksonville University, who has been studying exercise addiction for about 20 years, told Fox News. “It affects their drive and compels them. It becomes an obsession.”

Although most research has focused on the negative effects of not exercising enough (which includes and increased risk of obesity, heart disease and type 2 diabetes), some studies suggest ultramarathoners and other individuals who engage in vigorous long-term exercise see reduced heart function, Hausenblas said. In thin individuals, like Erin, over-exercising may also lead to high blood pressure, an increased risk of osteoporosis, and reproductive issues due to irregular menstruation.

How much is too much?

The Department of Health and Human Services recommends at least 150 minutes of moderate aerobic acticvity or 75 minutes of vigorous aerobic activity per week, or a combination of moderate and vigorous activity. But if you’re doing more than that, that doesn’t mean you’re an exercise addict, Hausenblas said.

“It’s not necessarily the amount someone is doing,” she explained. “You can have someone train for a race four or five hours a day, but for exercise addicts, there’s a different motivation behind it.”

Exercise addicts typically cancel social plans, and may skip or put off meals to exercise. Hausenblas said one patient she spoke to lost a job because they left work for what was meant to be a 30-minute run ended up lasting two hours instead.

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Exercising through pain is another symptom of exercise addiction.

“If you’re a regular exerciser and you’re sick, you’re OK taking a few days off,” Hausenblas said. “But someone who’s exercise addicted won’t take days off.”

Another exercise addict Hausenblas has interviewed claimed he bought three gym memberships — one for the morning, one for the afternoon, and one for the evening — “because he didn’t want the gym to see how much they were exercising.”

Trisha VanDusseldorp, an assistant professor of exercise science at Kennesaw State University, told Fox News that common physical signs of exercise addiction are: persistent fatigue, an increased heart rate of five beats or more while resting, dehydration, impaired immune function, changes in hormone concentrations, muscle soreness, inadequate nutrition, poor recovery, and, in severe cases, unintentional weight loss.

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Over-exercisers also suffer from classic signs of addiction such as withdrawal and relapse, which are well known among drug addicts and alcoholics.

Are you at risk?

Although exercise addiction is rare, you can take the following inventory to test whether you may be at risk, VanDusseldorp said:

  • Salience (when the habit begins to dominate thinking, lead to cravings, and affect social behavior)
  • Mood modification (if you experience arousing feelings described as a “buzz” or a “high” that may be numbing or give you a feeling of escape)
  • Tolerance (when you need to practice more of the same habit to achieve the former effects)
  • Withdrawal symptoms (when you have unpleasant feelings after reducing or stopping the activity)
  • Conflict (when the activity begins to negatively impact relationships between the addict and his or her family and friends, as well with as his or her activities)
  • Relapse (when you tend to repeat previous patterns of activity and resume the addiction, even after abstaining from it)